Neutron emissive target



United States Patent 3,205,564 NEUTRON EMISSIVE TARGET Jacques Shaw, Reading, Mass., assignor to High Voltage Engineering Corporation, Burlington, Mass, a corporation of Massachusetts N0 Drawing. Filed May 2, 1963, Ser. No. 277,495 15 Claims. (Cl. 29182.2)

The invention described and claimed herein relates generally to neutron emissive targets. There are comprehended various targets which, when fabricated in accordance with the principles of the invention, exhibit significant improvement over prior art devices in target life and operating characteristics. Such targets find particular utility when employed in conjunction with high current density ion beams having energies of the order of a million electron volts and greater.

The use of neutron emissive targets with high energyhigh intensity ion beams has heretofore been characterized by a rapid deterioration of the target material. This rapid deterioration is caused chiefly by blisters which form on the beam intercepting surface of the target. Such blisters form rapidly, and erupt and burn away to effect a continuing erosion which in a short time renders the target inoperable. The formation of blisters on the ion beam intercepting surface of the target is attributed to gas pressure resulting from ion recombination within the target material. That is, the high energy ions of the beam penetrate into the target material where they recombine with electrons to form gas molecules. When a sufiicient number of these gas molecules form in localized areas near the target surface the resulting pressure creates blisters which, when the pressure becomes sufficiently great, erupt and burn. The phenomenon of ion recombination is reviewed by Robert C. Frank in an article entitled Gases-in-Solids, published in International Scieince and Technology, September 1962, pp. 53-59.

The presence of occluded ion recombination gas in target material intercepting a high energy-high intensity ion beam has to date prevented the development of practicable long life neutron emissive targets. Conventional beryllium and lithium targets commonly become deteriorated after only a few hours of use with intense ion beams of one million electron volts and greater, and lose output in ten to twenty hours. The resulting excessive expense and out-of-operation time involved in frequent target replacement indicates the current need for improved long life neutron emissive targets. It is toward the fulfillment of this need that the present invention is directed.

Accordingly it is a principal object of this invention to provide a new and improved neutron emissive target that is suitable for use with high energy-high intensity beams.

It is another object of this invention to provide targets fabricated of powdered neutron emissive materials such as beryllium or lithium that have powder grain sizes and compacted densities adapted to permit the escape of ion recombination gas molecules therefrom.

It is yet another object of this invention to provide a long life neutron emissive target adapted to use with an ion beam having an energy greater than one million electron volts and a current density greater than one milliampere/cm? These, together with other objects and features of this invention will become readily apparent from the following detailed description of the novel target compositions comprehended by the invention. While specific materials and values are referred to herein, it is intended that they be taken as illustrative of the principles of the invention and not in a limiting sense.

The adverse effects of the above-described ion recom- 3,205,564 Patented Sept. 14, 1965 bination gas phenomenon are obviated by the novel neutron source target members of the present invention. It has been discovered, and it is a fundamental concept of this invention that targets fabricated of neutron emissive powder of certain grain sizes and compacted densities do not exhibit significant surface blistering and erosion. In recognition of the gas pressures being built up beneath the beam intercepting surface of conventional targets, the devices of the present invention have been'fabricated to permit the escape of such gas before pressures sufiicient to cause blistering can occur. This has been accomplished by compacting a finely powdered neutron emissive material such as beryllium or lithium, to a density that is substantially less than theoretical or ultimate density. A neutron emissive target that is to be used with a high energy ion beam, however, is subject to several requirements that dictate ranges from which the powder size and compacting densities may be chosen. These requirements are not necessarily compatible and some compromise is required to obtain an optimum result. For example, the requirements of effective heat conduction and good mechanical strength would suggest compacted densities of at least ninety-nine percent of theoretical, while efficient ion recombination gas escape capabilities are best obtained with much lower compacted densities. Also, minimum gas occlusion within individual grains can be achieved by using extremely small grain sizes. On the other hand, if the grain size is made too small, low compacted densities and adequate mechanical strength cannot be maintained.

In accordance with the above considerations, it has been determined that for neutron emissive materials such as beryllium and lithium, particle sizes in the range of five to fifteen microns, and compacted densities in the range of eighty to eighty-five percent of ultimate density may be employed to produce very satisfactory targets. Targets fabricated of neutron emissive material of grain sizes of less than three and greater than twenty microns and compacted to densities of less than seventy and greater than ninety-five percent of theoretical density are generally unsatisfactory.

Fabrication of the neutron emissive targets comprehended by this invention includes the steps of ball milling a neutron emissive material such as lithium or beryllium to a powder grain size in the range of three to twenty microns (preferably in the range of five to fifteen microns); fabricating the powdered material into a foil of the order of ten to forty mils and to a density in the range of seventy to ninety-five percent of theoretical density (preferably in the range of eighty to eighty-five percent of theoretical density); sintering the foil thus formed; cutting coupons of the desired target size out of the foil; and, brazing the coupons onto backing discs of high conductivity metal such as copper or silver. In addition to its conventional function as a heat sink, the backing disc of the present invention further operates to effect a vacuum tight closure of the vacuum system, thus ensuring against any loss of vacuum that might occur due to target porosity.

Having thus described the principles of the invention, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for the purposes of limitation, the scope of the invention being set forth in the following claims.

I claim:

1. A neutron emissive target for use in combination with a high energy ion beam consisting of a member fabricated of sintered powdered, neutron emissive material, said material having a grain size in the range of three to twenty microns and a compacted density in the range of seventy to ninety-five percent of ultimate density,

"I I) said member being adapted to elfect the escape of ion recombination gas molecules therefrom.

2. For use with an ion beam having an energy greater than one million electron volts and a current density greater than one milliampere/cmP, a neutron emissive target consisting of the combination of a member fabricated of powdered neutron emissive material, said powdered material having a grain size in the range of three to twenty microns and a compacted density in the range of seventy to ninety-five percent of ultimate density, and a metallic backing plate.

3. A neutron emissive target as defined in claim 1 in combination with a metallic backing plate.

4. A neutron emissive target as defined in claim 1 in combination with a copper backing plate.

5. A neutron emissive target as defined in claim 4 wherein said neutron emissive material comprises beryllium.

6. A neutron emissive target as defined in claim 2 wherein said neutron emissive material comprises beryllium.

7. A neutron emissive target as defined in claim 2 wherein said neutron emissive material comprises lithium.

8. A neutron emissive target as defined in claim 2 wherein said powdered material has a grain size in the range of five to fifteen microns.

9. A neutron emissive target as defined in claim 2 wherein said powdered material has a grain size in the range of eight to twelve microns.

10. A neutron emissive target as defined in claim 2 wherein said powdered material has a grain size of substantially ten microns.

I of substantially eighty-three percent of ultimate density.

14. A neutron emissive target as defined in claim 2 wherein said metallic backing plate is fabricated of copper.

15. A neutron emissive target as defined in claim 2 wherein said metallic backing plate is fabricated of silver.

References Cited by the Examiner UNITED STATES PATENTS 1,978,516 10/34 Weiger et al. 313330 2,549,596 4/51 Hamilton et al. 29182.3

OTHER REFERENCES Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, vol. 4, U.N., New York, 1956, page 93.

CARL D. QUARFORTH, Primary Examiner.

REUBEN EPSTEIN, Examiner. 

1. A NEUTRON EMISSIVE TARGET FOR USE IN COMBINATION WITH A HIGH ENERGY ION BEAM CONSISTING OF A MEMBER FABRICATED OF SINTERED POWDERED, NEUTRON EMISSIVE MATERIAL, SAID MATERIAL HAVING A GRAIN SIZE IN THE RANGE OF THREE TO TWENTY MICRONS AND A COMPACTED DENSITY IN THE RANGE OF THREE TO TWENTY MICRONS AND A COMPACTED DENSITY IN THE RANGE OF SEVENTY TO NINE-FIVE PERCENT OF ULTIMATE DENSITY, SAID MEMBER BEING ADAPTED TO EFFECT THE ESCAPE OF ION RECOMBINATION GAS MOLECULES THEREFROM. 